电子商务网站中用cookie保存最近浏览的商品只保存了一个产品

Question

                   HttpCookie cookie = new HttpCookie("pid");
                    List<string> list = new List<string>();//放商品id的集合
                    if (Request.Cookies["pid"] == null)
                    {
                        cookie.Value = id;
                        cookie.Expires = DateTime.Now.AddDays(1);
                        list.Add(id);
                        HttpContext.Current.Response.Cookies.Add(cookie);
                    }
                    else
                    {
                        //判断是否包含重复的id，重复的不加入list中
                        if (!list.Contains(id))
                        {
                            list.Add(id);
                        }
                        string aa = "";
                        foreach (string str in list)
                        {
                            aa += str + ",";
                        }
                        string bb = aa.Substring(0, aa.Length - 1);//这是list中所有id的字符串
                        cookie.Value = bb;
                        cookie.Expires = DateTime.Now.AddDays(1);
                        HttpContext.Current.Response.Cookies.Add(cookie);                    }